1. Technical Field of the Invention
The present invention relates generally to a recording apparatus such as a printer, a copying machine, or a facsimile machine which is designed to hold a transfer medium such as a printing sheet in a nip between an image carrier such as a photosensitive medium and a transfer member such as a transfer roller and apply a constant current to the transfer member to transfer visible images such as toner images formed on the image carrier to the transfer medium electrostatically.
2. Background Art
In recent years, as laser printers become increasingly more prevalent, there are increasing needs for compact structure and high-speed operation. Increased concerns about resources and environmental problems also require effective use of paper. From this point of view, printers are required to be capable of transferring images to both face and back of a print sheet. Further, with an increase in user convenience, there is an increasing need for printing images on various types of print mediums.
FIG. 6 shows one example of conventional printers.
The printer includes a brush electrifier 102, an exposure device 103, a developing device 104, a transfer device 105, a cleaner 106, a fixing device 107, a paper feeder 108, a paper ejector 109, and a reversing mechanism 110. The brush electrifier 102 has a rotary brush disposed in contact with a photosensitive drum 101 and rotates and applies the voltage to the rotary brush to electrify the photosensitive drum 101. The exposure device 103 activates a semiconductor laser in a given emission pattern to exposure the surface of the photosensitive drum 101 to laser beams to form an electrostatic latent image. The developing device 104 is of a non-magnetic component contact type and has disposed therein toner made of solid ink powder. The developing device 104 electrifies the toner and transport it to the surface of the photosensitive drum 101 to develop the electrostatic latent image. The transfer device 105 has a transfer roller 105 which forms a nip between itself and the photosensitive drum 101 through which transfer paper passes to transfer the toner image electrostatically. The cleaner 106 scrapes remaining toner off the photosensitive drum 101 after the image transferring operation. The fixing device 107 has a heat roller which fixes the toner image on the transfer paper with heat and pressure. The paper feeder 108 feeds the transfer paper to the transfer device 105. The paper ejector 109 ejects the transfer paper out of the fixing device 107. The reversing mechanism 110 turns over the transfer paper for transferring images on the back of the transfer paper.
In the image transferring operation, the photosensitive drum 101 is rotated at a given process speed. The brush electrifier 102 charges the surface of the photosensitive drum 101 at a black potential (several hundreds of minus volts).
Next, the semiconductor laser exposure device 103 is activated to emit light in a transfer pattern to form a desired latent image on the photosensitive drum 101. Specifically, charges are produced on laser-exposed portions of the photosensitive drum 101 so that the potential thereof drops to a level called a brightness potential, usually several tens of minus volts.
The toner charged negatively by the developing device 104 is applied on the photosensitive drum 101 through the developing roller to form a toner image. The developing roller made up of a stainless shaft covered with conductive rubber. The electrostatic transfer of the toner to the latent image to form the visible toner image is achieved by pressing the developing roller against the photosensitive drum 101 and applying several hundreds minus volts to the shaft of the developing roller to produce a strong electric field oriented to the latent image.
The photosensitive drum 101 and the transfer roller 105a of the transfer device 105 are, as described above, pressed on each other to form a nip (also referred to as a transfer nip below). The toner image moves to the transfer nip with rotation of the photosensitive drum 101. The transfer paper is transported from the paper feeder 108 to the transfer nip. A given current is supplied from a constant current source (not shown) to the transfer roller 105a to form the electric field between the transfer roller 105a and the photosensitive drum 101 so that the toner image is transferred electrostatically onto the transfer paper from the photosensitive drum 101.
The transfer roller 105a is made of a stainless shaft covered with a conductive foam such as a rubber resin) having a preselected surface-to-shaft resistance and a preselected hardness. Specifically, the transfer roller 105a is made of a flexible elastic member which has the preselected hardness at least on the surface thereof in order to increase an area of contact with the photosensitive drum 101. The transfer roller 105a is forced into constant engagement with the photosensitive drum 101.
The toner on the photosensitive drum 101 are charged negatively, therefore the polarity of the transfer current is positive. For instance, in a case where a print sheet of a certain size is transported in a lengthwise direction thereof to transfer a toner image thereto, the transfer current required for proper transfer is determined as a function of the process speed and the width of the print sheet and controlled constantly.
The toner remaining on the photosensitive drum 101 is removed by a urethane rubber-made cleaning blade installed in the cleaner 106 to clean the surface of the photosensitive drum 101 for the next operation.
The toner image on the print sheet is transported to the fixing device 107 and fixed by the heat and pressure. The print sheet is then ejected by the paper ejector 109.
Upon initiation of a back transfer mode of operation, the transfer paper is transported again to the transfer device 105 from the paper ejector 109 through the reversing mechanism 110. After an toner image is transferred onto the back of the transfer paper and fixed by the fixing device 107, the transfer paper is ejected by the paper ejector 109.
Such an image transferring apparatus using a transfer member like the transfer roller 105a has a drawback in that transferred image defects such as discharge-caused marks, lack of image density, or toner spots may arise during transfer of an image to the back of transfer paper or when the transfer paper is changed in type.
Some of transfer mediums have a greater change in electric resistance ranging over four figures depending upon a change in environmental condition. For example, thick paper containing cotton has usually a surface electrical resistance (i.e., sheet resistivity) of the order of 109 xcexa9/xe2x96xa1 and a volume resistivity of the order of 108 xcexa9/cm at high temperature and high humidity (e.g., 35xc2x0 C., 80% RH(Relative Humidity)), but they decrease to 1013 xcexa9/xe2x96xa1 and 1012 xcexa9cm, respectively, at low temperature and low humidity (e.g., 5xc2x0 C., 10% RH).
The problem of electrical resistance may occur during transfer of an image to the back of the transfer paper. This is because an image is transferred again to the transfer paper which has once passed through the fixing device 107. Specifically, some of transfer mediums require increasing the fixing temperature in order to provide a higher degree of fixing, thereby resulting in a great change in water content of the transfer mediums after the transfer of the image. This causes both the surface electrical resistance and the volume resistivity to increase in a few figures. Specifically, they will be 1014 xcexa9/xe2x96xa1 and 1014 xcexa9cm, respectively, at low temperature and low humidity (e.g., 5xc2x0 C., 10% RH).
A constant current control system may be used for power supply to the image transferring apparatus. In a case where a change in resistance of the transfer medium is relatively small, the constant current control system is capable of developing the voltage suitable for the image transfer regardless of environmental conditions. A great rise in resistance of the transfer medium due to the change in environmental condition or at the time of the back image transfer will, however, cause the transfer voltage to increase excessively, thereby increasing the possibility of the transferred image defects such as discharge-caused marks, lack of image density, or toner spots.
The transferred image defects are also caused by the compact structure of the device. Usually, the width of the photosensitive drum 101 and the transfer roller 105a is designed to be wider than that of a maximum effective width of the transfer mediums in view of a lateral shift of the transfer medium during transportation. This causes, as shown in FIG. 7, the transfer roller 105a to be flexed, thereby creating side areas of the transfer roller 105a which are in direct contact with the photosensitive drum 101 even when the transfer medium is held. The direct contact side areas have a low apparent resistance. When the transfer medium having a high resistance is transported to a transfer station and held thereat, an excessive current leaks from the transfer roller 105a to the photosensitive drum 101 through the direct contact side areas, thus eliminating an excessive rise in transfer voltage, resulting in a decrease in image transfer defect.
However, modern electrophotographic color printers are required to be reduced in size, thus increasing a need for decrease in width of the printers. This causes the width of side areas of the transfer roller 105a, as shown in FIG. 8, which are in direct contact with the photosensitive drum 101 to be decreased, thereby resulting in a decrease in leakage of the transfer current to the photosensitive drum 101, which leads to an excessive rise in transfer voltage so that a large number of transferred image defects occur.
In a case where the direct contact side areas of the transfer roller 105a are small, a rise in resistance of the transfer medium causes the current flowing through the direct contact side areas to be increased. In this case, the direct contact side areas are reduced in potential, so that fogs are produced on lateral ends of the photosensitive drum 101. A further increase in current flowing through the direct contact side areas causes the current to flow to the face of the transfer medium, thereby decreasing the potential of the photosensitive drum 101, making it difficult to rise the potential of the photosensitive drum 101 to a value required for the next electrifying process. This results in formation of fogs on the lateral ends of the transfer medium, causing print defects to occur.
The above described image defects may be analyzed in detail using an equivalent circuit model of the transfer station shown in FIG. 9. In the following discussion, C1 denotes a capacitance of the photosensitive drum 101 over the width thereof in contact with the transfer medium. C2 denotes a capacitance of the photosensitive drum 101 over the width thereof in direct contact with the transfer roller 105a. Rtn denotes an equivalent resistance of a toner image. Rp denotes the resistance of the transfer medium. Rt1 denotes the resistance of the transfer roller 105a over the width thereof in contact with the transfer medium. Rt2 denotes the resistance of the transfer roller 105a over the width thereof in direct contact with the photosensitive drum 101. It denotes a transfer current provided by a constant power supply. The transfer current It is divided into a current It1 contributing to the image transfer and a current It2 flowing from the direct contact area of the transfer roller 105a to the photosensitive drum 101. The relation between It1 and It2 is It1 greater than  greater than It2. Most of the transfer current It flows through the resistor Rt1. Thus, Rt2 greater than  greater than Rt1.
A rise in resistance of the transfer medium caused by a change in ambient condition or the image transfer to the back of the transfer medium is equivalent to a rise in resistance Rp in the equivalent circuit. Because of a rise in resistance of the whole of the circuit, the transfer voltage applied to the toner image resistance Rtn, thereby increasing the possibility of the discharge-caused marks during the image transfer.
The image defects caused by the decrease in width of the printer may be explained using the equivalent circuit of FIG. 9. The resistance Rt2 is increased as compared with the case where the width of the printer is greater, thus decreasing the current It2. This causes the current It1 to increase to increase the voltage applied to the toner image resistance Rtn, thereby increasing the possibility of the discharge-caused marks during the image transfer.
In the printer whose width is smaller (i.e., Rt2 is smaller), an increase in resistance Rp of the transfer medium causes the current It2 flowing directly to the photosensitive drum 101 to increase by a decreased amount of the current It1, thereby resulting in a decrease in potential at contact side areas of the photosensitive drum 101 and the transfer roller 105a, so that fogs are formed on the lateral ends of the photosensitive drum 101. A further increase in current will cause the current to flow through the photosensitive drum 101 toward the face of the transfer medium, thereby resulting in the formation of the fogs on the lateral ends of the transfer medium.
The conventional printers use a temperature/humidity sensor to control the transfer current for absorbing a variation in resistance of the transfer medium. This, however, results in complexity of the structure and increase in production costs.
In order to avoid the above problems, Japanese Patent First Publication No. 10-207258 proposes a bypass circuit connected directly to ground. This structure, however, causes most of the transfer current to flow to the bypass circuit, which impinges upon the transfer of images.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a recording apparatus designed to have a compact structure without producing a variation in resistance of a transfer medium caused by a change in ambient condition or the image transfer to the back of the transfer medium and forming any transferred image defects.
According to one aspect of the invention, there is provided a recording apparatus which comprises: (a) an image carrier; (b) a transfer member disposed in contact with the image carrier to hold a transported transfer medium therebetween, a constant current being applied to the transfer member to transfer visible images formed on the image carrier to the transfer medium; and (c) a bypass circuit having a given resistance. The bypass circuit is connected to portions of the transfer member which are in contact with the image carrier and located outside a range of passage of the transfer medium to cause the constant current supplied to the transfer member to flow partially to the bypass circuit, thereby avoiding formation of any print defects such as discharge-caused marks and developer spots on the transfer medium. The activities of the resistor serve to suppress excessive flow of the current to the bypass circuit.
According to another aspect of the invention, there is provided a recording apparatus which comprises: (a) an image carrier; (b) a transfer member disposed in contact with the image carrier to hold a transported transfer medium therebetween, a constant current being applied to the transfer member to transfer visible images formed on the image carrier to the transfer medium; and (c) a conductive member disposed in contact with a surface of the transfer member. The conductive member is connected to ground through a resistor.
In the preferred mode of the invention, a current equivalent to an amount of current leaking to the resistor from the conductive member is added to the constant current applied to the transfer member.
The conductive member is located outside a range of passage of the transfer medium through a nip between the image carrier and the transfer member.